Our objective is to develop new and highly specific chromosome banding techniques, including some in which staining is restricted to one or at most a few bands. Our approach is based on the assumption that particular nucleotide sequences, or families of sequences, have a nonrandom distribution along individual chromosomes. Selective denaturation of one family of sequences by a procedure which leaves others in the native configuration will be combined with an indirect immunoperoxidase technique to detect the denatured sequences, using antinucleoside antibodies which react with their specific antigenic determinants in single-stranded or denatured DNA but not in native DNA. Greater specificity will be sought by using monoclonal antibodies produced by hybridomas. In addition, antibodies of several different specificities, e.g. antibodies reactive with adenosine, cytosine, 5-methylcytosine or oligonucleotides of length 2 or 3, will be used. Immunocytochemical probes for detecting inactive and active gene clusters will also be sought, using antibodies to 5-methylcytidine and to nonhistone chromosomal proteins HMG14 or 17, respectively. The various probes will find applications in the study of chromosome organization and function, and in clinical and comparative cytogenetics.